[clang][Interp] Implement align builtins

__builtin_is_aligned
__builtin_is_align_up
__builtin_is_align_down

Author: tbaederr

clang/lib/AST/Interp/InterpBuiltin.cpp

@@ -977,6 +977,117 @@ static bool interp__builtin_complex(InterpState &S, CodePtr OpPC,
   return true;
 }
 
+/// __builtin_is_aligned()
+/// __builtin_align_up()
+/// __builtin_align_down()
+/// The first parameter is either an integer or a pointer.
+/// The second parameter is the requested alignment as an integer.
+static bool interp__builtin_is_aligned_up_down(InterpState &S, CodePtr OpPC,
+                                               const InterpFrame *Frame,
+                                               const Function *Func,
+                                               const CallExpr *Call) {
+  unsigned BuiltinOp = Func->getBuiltinID();
+  unsigned CallSize = callArgSize(S, Call);
+
+  PrimType AlignmentT = *S.Ctx.classify(Call->getArg(1));
+  const APSInt &Alignment = peekToAPSInt(S.Stk, AlignmentT);
+
+  if (Alignment < 0 || !Alignment.isPowerOf2()) {
+    S.FFDiag(Call, diag::note_constexpr_invalid_alignment) << Alignment;
+    return false;
+  }
+  unsigned SrcWidth = S.getCtx().getIntWidth(Call->getArg(0)->getType());
+  APSInt MaxValue(APInt::getOneBitSet(SrcWidth, SrcWidth - 1));
+  if (APSInt::compareValues(Alignment, MaxValue) > 0) {
+    S.FFDiag(Call, diag::note_constexpr_alignment_too_big)
+        << MaxValue << Call->getArg(0)->getType() << Alignment;
+    return false;
+  }
+
+  // The first parameter is either an integer or a pointer (but not a function
+  // pointer).
+  PrimType FirstArgT = *S.Ctx.classify(Call->getArg(0));
+
+  if (isIntegralType(FirstArgT)) {
+    const APSInt &Src = peekToAPSInt(S.Stk, FirstArgT, CallSize);
+    APSInt Align = Alignment.extOrTrunc(Src.getBitWidth());
+    if (BuiltinOp == Builtin::BI__builtin_align_up) {
+      APSInt AlignedVal =
+          APSInt((Src + (Align - 1)) & ~(Align - 1), Src.isUnsigned());
+      pushInteger(S, AlignedVal, Call->getType());
+    } else if (BuiltinOp == Builtin::BI__builtin_align_down) {
+      APSInt AlignedVal = APSInt(Src & ~(Align - 1), Src.isUnsigned());
+      pushInteger(S, AlignedVal, Call->getType());
+    } else {
+      assert(*S.Ctx.classify(Call->getType()) == PT_Bool);
+      S.Stk.push<Boolean>((Src & (Align - 1)) == 0);
+    }
+    return true;
+  }
+
+  assert(FirstArgT == PT_Ptr);
+  const Pointer &Ptr = S.Stk.peek<Pointer>(CallSize);
+
+  unsigned PtrOffset = Ptr.getByteOffset();
+  PtrOffset = Ptr.getIndex();
+  CharUnits BaseAlignment =
+      S.getCtx().getDeclAlign(Ptr.getDeclDesc()->asValueDecl());
+  CharUnits PtrAlign =
+      BaseAlignment.alignmentAtOffset(CharUnits::fromQuantity(PtrOffset));
+
+  if (BuiltinOp == Builtin::BI__builtin_is_aligned) {
+    if (PtrAlign.getQuantity() >= Alignment) {
+      S.Stk.push<Boolean>(true);
+      return true;
+    }
+    // If the alignment is not known to be sufficient, some cases could still
+    // be aligned at run time. However, if the requested alignment is less or
+    // equal to the base alignment and the offset is not aligned, we know that
+    // the run-time value can never be aligned.
+    if (BaseAlignment.getQuantity() >= Alignment &&
+        PtrAlign.getQuantity() < Alignment) {
+      S.Stk.push<Boolean>(false);
+      return true;
+    }
+
+    S.FFDiag(Call->getArg(0), diag::note_constexpr_alignment_compute)
+        << Alignment;
+    return false;
+  }
+
+  assert(BuiltinOp == Builtin::BI__builtin_align_down ||
+         BuiltinOp == Builtin::BI__builtin_align_up);
+
+  // For align_up/align_down, we can return the same value if the alignment
+  // is known to be greater or equal to the requested value.
+  if (PtrAlign.getQuantity() >= Alignment) {
+    S.Stk.push<Pointer>(Ptr);
+    return true;
+  }
+
+  // The alignment could be greater than the minimum at run-time, so we cannot
+  // infer much about the resulting pointer value. One case is possible:
+  // For `_Alignas(32) char buf[N]; __builtin_align_down(&buf[idx], 32)` we
+  // can infer the correct index if the requested alignment is smaller than
+  // the base alignment so we can perform the computation on the offset.
+  if (BaseAlignment.getQuantity() >= Alignment) {
+    assert(Alignment.getBitWidth() <= 64 &&
+           "Cannot handle > 64-bit address-space");
+    uint64_t Alignment64 = Alignment.getZExtValue();
+    CharUnits NewOffset =
+        CharUnits::fromQuantity(BuiltinOp == Builtin::BI__builtin_align_down
+                                    ? llvm::alignDown(PtrOffset, Alignment64)
+                                    : llvm::alignTo(PtrOffset, Alignment64));
+
+    S.Stk.push<Pointer>(Ptr.atIndex(NewOffset.getQuantity()));
+    return true;
+  }
+
+  // Otherwise, we cannot constant-evaluate the result.
+  S.FFDiag(Call->getArg(0), diag::note_constexpr_alignment_adjust) << Alignment;
+  return false;
+}
+
 bool InterpretBuiltin(InterpState &S, CodePtr OpPC, const Function *F,
                       const CallExpr *Call) {
   const InterpFrame *Frame = S.Current;
@@ -1291,6 +1402,13 @@ bool InterpretBuiltin(InterpState &S, CodePtr OpPC, const Function *F,
       return false;
     break;
 
+  case Builtin::BI__builtin_is_aligned:
+  case Builtin::BI__builtin_align_up:
+  case Builtin::BI__builtin_align_down:
+    if (!interp__builtin_is_aligned_up_down(S, OpPC, Frame, F, Call))
+      return false;
+    break;
+
   default:
     S.FFDiag(S.Current->getLocation(OpPC),
              diag::note_invalid_subexpr_in_const_expr)